Is the 0.6 wind load reduction(ASD) just to check deflection?

[structure567]

I am designing a cantilever steel column with a solid sign and have not found an answer if I can apply the 0.6 wind load reduction for thr strength design. I know ASCE shows the ultimate wind speed and applying .6 converts it to nominal wind speed. Am I to use the .6 reduction just for deflection or can it be applied to design the strength of the steel column as well?

 

[phamENG]

Read the commentary on serviceability issues. Wind for deflection is often taken as the 10 year MRI, which is roughly 0.42*W. The 0.6*W is only applicable to ASD load combinations.

 

[structure567]

Read the commentary on serviceability issues. Wind for deflection is often taken as the 10 year MRI, which is roughly 0.42*W. The 0.6*W is only applicable to ASD load combinations.

Thank you for your reply. So wind load reduction is actually .42 for deflection and not .6? I was confused because I thought .6 was the reduction for deflection check up only. Thank you.

 

[phamENG]

No problem. 0.6 is for checking strength/stress under ASD load combinations. Determination of your MRI for serviceability checks is especially important when you start looking at risk category 3 and 4 structures. But if this is for a house or typical office building, 10 year MRI is usually good.

Take a look at the IBC in chapter 16 in the table for allowable deflections. I believe there's a footnote that discusses this.

 

[structure567]

The cantilever column is for just to place a 7' by 7' sign. It is not a high risk category.

 

[TheDW]

It is my understanding that the 0.42 factor that they let you take for deflection comes from a 0.7 factor to convert to 10 year MRI, combined with a 0.6 factor for the ASD load combinations that you would use for serviceability checks. 0.7 * 0.6 = 0.42.

Also, the 0.7 conversion is only applicable when using C&C loads, not MWFRS, per the table phamENG is talking about. C&C = 0.7*0.6, MWFRS=0.6

Someone please correct me if I'm wrong, the wind speed provision factors are pretty obnoxious.

Edit: to answer the main question OP. If you're doing LRFD strength design, do not take a 0.6 reduction. Only when checking deflection.

 

[phamENG]

DayWalker - thanks for that clarification. You are correct. It's C&C loading, not MWRFS. And you're right on the MRI conversions. The 0.6 converts from the 700 year MRI used for LRFD to the 50 year MRI used in ASD checks. The 0.7 is a conversion from 50 year MRI to 10 year MRI. So the conversion from 700 to 10 is 0.7*0.6=0.42.

 

[structure567]

Edit: to answer the main question OP. If you're doing LRFD strength design, do not take a 0.6 reduction. Only when checking deflection.

daywalker- Are you saying that the .6 is to be used only for deflection checks(ASD)? 0.6 reduction should not be used in strength design(ASD)?

 

[phamENG]

ASD is not just for deflection checks. ASD is to check the capacity of the member (allowable stress for wood, allowable strength for steel) against the expected load. If you're using ASD, use the load combinations in ASCE 7 2.4 or whatever your governing code is.

 

[JAE]

structure567 - you need to FIRST decide if you are designing the steel column based on LRFD or ASD....your choice.

If you choose ASD, then with modern US codes the wind forces are "ultimate" - i.e. wind speed is perhaps 115 mph.
This means that to design with ASD, you need to use a 0.6 load factor on the wind.
With 0.6W you would then design for ALLOWABLE strengths given in AISC for ASD design (i.e. you would use an Ω safety factor).
To check deflections, with C&C wind, you could then take 0.7 of that wind load (0.6)(0.7)W to check deflections.

If you choose LRFD, the the wind load would be 1.0W.
You would design your steel column with AISC using that 1.0W load based on an ultimate strength of the steel (i.e. you would use a Φ factor to reduce your strength capacity (ΦRn).
Then to check deflections - you would use (0.6)(0.7)W again as for ASD.

 

[azcats]

Whether or not to use the 0.6W depends on (as phamENG noted) whether your using the ASD design per AISC. If you're using LRFD you need to use the strength design load combinations with 1.0W.

ETA - JAE beat me to it.

 

[klaus.]

Please let me summarize If I understand correct.

Given is the ultimate wind load according to latest ASCE Wind Code
lets call it W

LRFD Design of C&C and MWFRS use : 1.0 W
ASD design of C&C and MWFRS use : 0.6 W
Deflection check of MWFRS use : 0.6 W
Deflection check of C&C use : 0.7 x 0.6 x W = 0.42 W

Is this correct ??

 

[slickdeals]

Deflection check of MWFRS can be done utilizing the appropriate wind speed from the maps found in the commentary of the code. They provide values for 10-year and 25-year events. Typically deflections are calculated at 10 or 25 year return periods.
Multiply the deflection you calculate for MWFRS using [V(10 or25)^2/Vult^2)]